A tappet lubricating device has been previously proposed as a lubricating device of an internal combustion engine, which has a housing that includes a receiving chamber where oil for lubricating respective lubricating portions of a drive mechanism is retained. In the tappet lubricating device, oil for lubricating respective lubricating portions of a plunger drive mechanism is retained in a tappet receiving chamber of a housing of a supply pump installed to an internal combustion engine.
Here, the plunger drive mechanism is a converting mechanism that converts rotation of a cam of a camshaft into reciprocation of a plunger and includes a tappet (used as a lifter), a roller of the tappet, and a roller pin (a rotational shaft of the roller) (see, for example, JP2008-286124A).
FIG. 5 indicates a structure of a typical plunger pump that is used as a supply pump having the above described plunger drive mechanism.
The supply pump is constructed to pressurize and pump fuel, which is drawn into a pressurizing chamber 105 through a fuel suction valve 104, when a plunger 103 is reciprocated in a cylinder 102 along a contour of a cam 101 of a camshaft.
In the plunger drive mechanism, when the cam 101 is rotated through rotation of the camshaft, a roller pin 107 and a tappet 108 are reciprocated through a roller 106 in a top-to-bottom direction in the drawing. The upward and downward movement of the tappet 108 described above is transmitted to the plunger 103, so that the plunger 103 is reciprocated in the top-to-bottom direction of the drawing in the cylinder 102.
In the supply pump, slide movement occurs at a contact portion between the plunger and the tappet. Therefore, in order to limit wearing and seizing at the contact portion between the plunger and the tappet, oil is circulated and supplied in a tappet receiving chamber of the plunger drive mechanism.
For example, the oil is supplied to the contact portion between the plunger 103 and the tappet 108, so that an oil film is formed between the plunger 103 and the tappet 108 to lubricate the contact portion between the plunger 103 and the tappet 108.
The oil is also supplied to a contact portion between the cam 101 and the roller 106, a contact portion between the roller pin 107 and the tappet 108, and a roller bush placed between the roller 106 and the tappet 108.
Furthermore, a seal member 110 is installed in the supply pump to seal between the pressurizing chamber 105, which is formed at one axial end of the cylinder 102, and a tappet upper side chamber 109 of a tappet receiving chamber.
In the supply pump, as shown in FIG. 5, when the roller 106 is raised along the cam 101, the tappet 108 is moved upward along with the plunger 103. When the tappet 108 is moved upward, a volume of the tappet upper side chamber 109 of the tappet receiving chamber, which is located on the plunger side of the tappet 108, is reduced. In this way, the oil, which is retained in the tappet upper side chamber 109, is compressed, so that a pressure in the tappet upper side chamber 109 is increased.
Therefore, the seal member 110, which liquid-tightly seals between the tappet upper side chamber 109 and the pressurizing chamber 105 through a clearance, may be damaged, or a roller contact surface 111 of the cam 101 may be damaged. Thus, there will be a disadvantage, such as deterioration in a sealing performance of the seal member 110 or wearing of the roller contact surface 111 of the cam 101.
In order to avoid the above disadvantage, a pressure release passage, which releases a pressure (an oil pressure) in the tappet upper side chamber 109, is required. However, in many cases, a sufficient flow passage cross-sectional area of the pressure release passage cannot be ensured due to interference between the pressure release passage and another component or a fuel flow passage.
In view of the above disadvantage, JP2008-286124A discloses a supply pump that has a communication oil passage, which communicates between the tappet upper side chamber and a cam/shaft receiving chamber and is formed in a tappet guide of a housing as a pressure release passage for releasing the pressure in the tappet upper side chamber.
In this case, the communication oil passage, which is communicated with the tappet upper side chamber, opens in a bottom surface, which is different from a tappet slide surface of the tappet guide, to form a large space of the cam/shaft receiving chamber. In this way, the amount of change in the volume of the cam/shaft receiving chamber relative to the amount of change in the volume of the tappet upper side chamber becomes very small. Thereby, the pressure of the tappet upper side chamber cannot be easily decreased, so that oil suctioning effect for suctioning the oil from the tappet upper side chamber into the cam/shaft receiving chamber is small.
Thus, in the supply pump of JP2008-286124A, the oil suctioning effect, which is exerted by the upward movement of the tappet, cannot be expected too much, and the pressure increase of the tappet upper side chamber cannot be sufficiently limited.